At the present time solar cells are classified into three generations, which are described below.
First-generation solar cells are silicon-based solar cells that dominate the solar market (80 to 90%). Solar cells of this type are manufactured of monocrystalline or polycrystalline silicon, and, in spite of high manufacturing cost (typically ranging from $3/W to $5/W which is much higher than is required for wide implementation), popularity of these solar cells results from their high efficiency, well developed processing, and practically unlimited availability of silicon.
Solar cells of the second generation are also known as thin-film solar cells. The cells of this type are less expensive, lighter in weight, and more attractive in appearance than solar cells of the first generation. However, they are less efficient than first-generation cells.
Third-generation solar cells do not need the p-n junction necessary in traditional silicon-based and thin film cells. Third-generation cells contain a wide range of potential solar innovations, including polymer solar cells, nanocrystalline, nanomaterial-based cells, and dye-sensitized solar cells.
Irrespective of a provision of later generations, interest in solar cells of the first generation remains very keen, and research in this direction continues. The high fabrication cost of the first-generation solar cells results mainly from several high-temperature processes required to form functional p-n junctions, barrier layers, passivation and contact regions, emitters and selective emitters, back-surface field (BSF) regions, which are required on front-surface modifications, and front-surface field (FSF) regions, which are required on back-surface modifications, etc.
The aforementioned processes are typically performed in high-temperature thermal diffusion furnaces, belt furnaces, and rapid thermal annealing (RTA) chambers. Diffusion and annealing processes are generally power-consuming and time-consuming, and equipment with which these processes are carried out generally requires periodic calibration, testing, and maintenance. Another source of complexity and cost increase in the manufacture of first-generation solar cells is patterning, a process that typically involves the use of photolithography for forming selective emitters, contact regions, electrodes, and other cell elements.
Attempts have been made to simplify fabrication of silicon-based solar cells, e.g., by reducing the number of masking, diffusion, and passivation steps, which are used in screen printing or jet printing with consecutive annealing of screen-printed layers. For example, conductive electrodes can be formed by the screen-printing technique on both sides of a solar cell (for front-side screen-printing and annealing, refer to “Crystalline and thin-film silicon solar cells: state of the art and future potential” by Dr. Martin A. Green, Solar Energy, Vol. 74, pp. 181 to 192 (2003) and for back-side screen-printing and annealing, refer to U.S. Patent Application Publication No. 20090025786, published on Jan. 29, 2009, inventors: Ajeet Rohatgi, et al).
U.S. Patent Application Publication No. 20100012185 (published on Jan. 21, 2010; inventors: Christian Schmid, et al) and U.S. Pat. No. 6,262,359 issued on Jul. 17, 2001 to Daniel Meier, et al, describe a process wherein aluminum or aluminum-containing paste is deposited on the back side of a solar cell and is annealed to create a back-surface field (BSF) region without performing a thermal diffusion step.
Some known methods offer formation of conductive electrodes by applying aluminum- or silver-containing paste by means of a screen-printing process and then melting the paste for penetration thereof through dielectric passivation layers for forming contacts directly on the front-side N-type emitters of the cell (see e.g., U.S. Patent Application Publication No. 20090044858 published on Feb. 19, 2009, inventors: Yueli Y. Wang, et al) or by forming a BSF region and a back side contact of the solar cell (see U.S. Patent Application Publication No. 20100098840 published on Apr. 22, 2010; inventor Chen-Hsun Du, et al, and U.S. Pat. No. 6,695,903 issued on Feb. 24, 2004 to Armin Kubelbeck, et al).
It is known in the art to apply a dopant paste onto a substrate, e.g., by screen printing, and then to use the dopant paste to form selectively doped regions in the Si substrate. For example, U.S. Pat. No. 6,825,104 issued on Nov. 30, 2004 to J. Horzel, et al, describes a method of manufacturing a semiconductor device wherein a pattern of solid dopant is selectively applied to the surface of a semiconductor substrate after which the dopant atoms are diffused from the solid dopant source into said substrate to form a first diffused region by controlled heat treatment in a gaseous environment surrounding the semiconducting substrate. At the same time, the dopant source is diffused into the substrate indirectly by means of said gaseous environment, whereby a second diffusion region is formed at least in some areas of the substrate not covered by the pattern. In the final stage a metal contact pattern is formed substantially in alignment with the first diffusion region without substantial etching of the second diffusion region.
U.S. Pat. No. 6,429,037 issued on Aug. 6, 2002 to Stuart R. Wenham, et al, discloses a method for forming selective emitters without recourse to a conventional diffusion step generally required for the formation of heavily doped regions of selective emitters. This is achieved by means of laser-assisted local heating of a dopant source that also serves as a passivation layer and mask for consequent metallization. The method also allows formation of self-aligned contacts on selective emitter regions. This method has some advantages; however, it requires at least one thermal diffusion operation, complex optimization of the laser operation, and, potentially, additional deposition and annealing steps.
Another efficient attempt to minimize the number of diffusion, passivation, and masking operations in solar cell fabrication is disclosed in U.S. Pat. No. 7,615,393 issued on Nov. 10, 2009 to Sunil Shah, et al. The method described in this patent provides a substrate that is doped with boron and includes a first substrate surface with a first surface region and a second surface region. A first set of nanoparticles, which includes a first dopant, is deposited on the first surface region. The substrate is heated in an inert ambient to a first temperature, whereby a first densified film is created, and then a first diffused region is formed with the first diffusion depth in the substrate beneath the first surface region. The method also includes exposing the substrate to a diffusion gas that includes phosphorous at a second temperature for forming a phosphosilicate glass (PSG) layer on the first substrate surface, and then a second diffused region with a second diffusion depth is formed in the substrate beneath the second surface region wherein the first diffused region is proximate to the second diffused region. The method further includes exposing the substrate to an oxidizing gas at a third temperature, wherein an SiO2 layer is formed between the PSG layer and the substrate surface, wherein the first diffusion depth is substantially greater than the second diffusion depth. Thus, multidoped junctions are formed on a substrate essentially without photolithography.
While this method represents an interesting advance toward simplification of solar cell manufacturing, it still requires at least one complex thermal diffusion process (step that includes using a dopant gas). Also, diffusion of phosphorus onto the front surface is conducted simultaneously with diffusion of aluminum onto the back side, which may cause uncontrolled doping on the back-side doped regions. Furthermore, this method requires alignment of the metal electrodes to the doped selective emitter (front side) and BSF regions (back side), which is not done automatically and which may involve additional steps.
Bulgarian Patent No. BG109881 issued on Dec. 30, 2008 to Petko Vitanov, et al, describes a solar cell with a field-induced emitter in the form of an inversion layer wherein the front-side emitter is formed by an electric field generated by an electric charge developed in a dielectric antireflective coating on the front surface of the solar cell. However, this type of cell requires formation of selective N+ doped emitters and BSF regions (needed to provide contact regions for photocurrent) by means of conventional high-temperature diffusion.
Analysis of prior art shows that although a significant reduction in number of masking, diffusion, and passivation operations has been achieved in the manufacture of solar cells, none of the existing methods eliminate the thermal diffusion and masking steps required to form emitters, selective emitters, BSF, FSF, self-aligned electrodes, and other solar cell elements. Therefore, the cost of manufacturing silicon-based solar cells remains relatively high.